What perspective is the author telling the story from?

The book I was reading when I chose this question is The Amulet of Samarkand and it is written by the author Jonathan Stroud. In this book Mr. Stroud changes between 2 different perspectives constantly. The first of the 2 perspectives is the young magician's apprentice, Nathaniel Underwood's perspective, and the second is the djiin, Bartimeus of Uruk's perspective. Bartimeus is Nathaniel's personal demon slave, so from his perspective, magicians are evil and malicious, since they are always imprisoning djinn and forcing them to do things. But from Nathaniel's point of view, the magicians are constantly protecting everyone from the evil djinn. So the two are always trying to prove that their side is right. So you can't really say that he is telling it from one perspective, and that makes all the difference of a good story, and a not-so-good one.